The purpose of this research is to elucidate the pathophysiologic mechanism responsible for ventricular enlargement in obstructive hydrocephalus using quantitative information contained in CT scans. Since the adult cranial cavity has a rigidly defined volume, ventricular enlargement must occur at the expense of other intracranial spaces. Two competing hypotheses attempt to explain the mechanism allowing for ventricular expansion: 1) that brain water is lost and 2) that brain tissue is lost. Using quantitative information contained in CT scans, these hypotheses will be tested. An increase in brain density will be noted as a result of water loss, while a decreased density will result from tissue loss. The findings from this study bear directly on the reversibility of obstructive hydrocephalus in man.